Various image projection apparatuses which project images using a laser beam have been proposed. Since the laser beam has a very narrow spectral width, it is possible to provide a display having a wide range of color reproduction by using three colored lasers of red, blue, and green.
Since laser light has a strong directivity and its energy density can be heightened, it is possible to perform beam-formation of the light from the laser source and scan the laser beam by a compact scanning device. Therefore, there is a possibility of realizing a compact image projection apparatus. For example, Japanese Patent Laid-Open No. H07-151995 has disclosed a laser scanning display in which a scanning device scans a laser beam to display images on a screen as an image projection apparatus which projects images using a laser beam.
There is a MEMS(Micro Electro Mechanical System) device which is manufactured using semiconductor manufacturing techniques as a scanning device; the MEMS device can operate at a high speed while it is a compact and lightweight device.
On the other hand, since the laser beam has high coherence, an interference pattern, which is called a “speckle noise” and caused by the roughness of the screen, appears when the laser beam is projected on the screen. The interference pattern causes degradation of sharpness of the image displayed by the image projection apparatus.
A method for reducing the speckle noise has been disclosed in Japanese Patent Laid-Open No. 2000-206449, in which a transparent optical element having a refraction index of “n” and including “N” areas whose thicknesses change by “Δt” is used. In the method, laser light that is a diverging luminous flux from a semiconductor laser (laser diode) is converted into a parallel luminous flux by a collimator lens, and enters the transparent optical element. The transparent optical element gives optical path differences “(n-1)Δt” to luminous fluxes (divided luminous flux) passing through portions with different thicknesses, thereby reducing the coherence of each divided luminous flux. It is possible to reduce the speckle noise by overlapping the incoherent luminous fluxes that have passed through the transparent optical element by a lens.
Further, a method for reducing the speckle noise has been disclosed in Japanese Patent Laid-Open No. H06-208089, in which a laser beam is scattered with a rotatable diffusing element. The speckle pattern is changed at a speed undetectable to human's eyes by the rotation of the diffusing element. Overlapping the speckle patterns changing at a high speed by an afterimage effect of the human's eye makes it possible to reduce the speckle noise.
Moreover, a method for reducing the speckle noise has been disclosed in “Applied Optics/Vol.37, No.10/1 April 1998 ‘Speckle reduction in laser projection system by diffractive optical elements’”, in which a diffraction grating is used and the diffracted light components are overlapped.
Furthermore, a laser display with a one-dimensional diffractive light modulator has been disclosed in U.S. Pat. No. 6,323,984. In the laser display, a phase modulator is arranged at a position conjugated with a light modulator to change the phase of the light. Thereby, the speckle noise is reduced.
However, in the method disclosed in Japanese Patent Laid-Open No. 2000-206449, it is necessary to increase the step “Δt” of the transparent optical element to give enough optical path differences to the divided luminous fluxes. As a result, it becomes difficult to downsize the optical system. In particular, the size of the transparent optical element becomes larger when using light having a long coherence length such as a solid-state laser and a single-mode semiconductor laser.
Further, in the method disclosed in Japanese Patent Laid-Open No. H06-208089, the loss of light amount becomes relatively large because the laser beam is transmitted through the diffusing element (frosted glass).
Moreover, in the method disclosed in U.S. Pat. No. 6,323,984, the optical system becomes complex because the phase modulator is arranged at the position conjugated with the light modulator.
Furthermore, in the methods disclosed in Japanese Patent Laid-Open No. H06-208089 and “Applied Optics/Vol.37 No.10/1 April 1998”, a spatial light modulator is required for projecting an image of a two-dimensional or one-dimensional light modulator.